Absorption refrigeration systems are well known for large and small scale cooling, including air conditioning, utilizing an absorption pair of a volatile refrigerant such as ammonia or various fluorocarbons and a liquid absorbent. Representative absorption pairs are ammonia (as refrigerant) and water (as absorbent), water (as refrigerant) and lithium bromide (as absorbent) and dichloromonofluoromethane (as refrigerant) and the dimethylether of tetraethylene glycol (as absorbent). Refrigeration with such systems depends upon withdrawing heat into the refrigerant in an evaporator.
Absorption systems have been suggested for heating applications as well, wherein the evaporator is used to withdraw heat from a heat source such as outside air. The heat outputs of the absorption system, principally the absorber and the condenser, are then used to provide heat for space heating. U.S. Pat. Nos. 3,527,060, 3,527,061 and 3,638,452, all to Kruggel, are representative of such systems based upon the use of ammonia and water. U.S. Pat. Nos. 4,106,309, 4,127,009, 4,127,010 and 4,127,993, all to Phillips, describe an absorption heat pump system, particularly for use with fluorocarbon refrigerants and furan-derivative absorbents. Particular attention is drawn to FIGS. 4, 5 and 6 of U.S. Pat. No. 4,127,010, which illustrate the different pressure and temperature conditions of the absorption refrigeration cycle when the ambient air is approximately 47.degree. F. (8.degree. C.) and 0.degree. F. (-18.degree. C.) in the heating mode and 95.degree. F. (35.degree. C.) in the cooling mode. In particular, as cold ambient air conditions are encountered, the evaporator temperature and the low side pressure in the evaporator and absorber drop, the refrigerant content of both the rich liquor leaving the absorber and the weak liquor returning to the absorber decrease and the peak boiler temperature in the generator increases. While these variations assist in adapting the absorption system to such cold ambient conditions, it has now been found that even with such adjustments, difficulties can arise under cold ambient conditions. In particular, the heating capacity of the absorption system falls off as ambient conditions get colder, while at the same time, the weak liquor at the hottest part of the boiler may become excessively hot. Because, in general, less heat may be taken into the absorption system through the evaporator from cold ambient air, it would be desirable to increase the heat input into the system from the boiler.